Patent GB-1469984-A (GB)
Title: Temperature dependent voltage reference circuit
IPC: G01R1/28
Filed: 1974-08-22
Abstract: 1469984 Measuring temperature electrically RCA CORPORATION 22 Aug 1974 [27 Aug 1973] 36927/74 Heading G1N A circuit for producing a temperature-dependent voltage between first and second input terminals comprises: means for connecting a source of operating current between the input terminals; first and second bi-polar transistors of the same conductivity type arranged to operate at the temperature of interest; means for conducting a portion of the operating current to the joined emitters of the transistors; means responsive to the voltage between the input terminals to apply a potential difference between the bases of the transistors so as to cause them to conduct with unequal current densities through their respective base-emitter junctions; means responsive to the collector currents to vary the input-terminal voltage so that the resultant p.d. between the transistor bases maintains their current densities substantially in a predetermined ratio. Transistors 19, 18 Fig. 1 having base/emitter junction areas in the ratio 16:4, as shown by encircled figures, are arranged as a common-emitter differential amplifier, their bases being connected respectively to points 13, 14 of a potential divider 15-17 arranged between a pair of terminals 11, 12. The collector current of transistor 19, inverted by a unity-gain current-mirror amplifier 21 comprising matched transistors 22, 23, is compared with the collector current of transistor 18 at the base of a transistor 25 forming part of a high-gain amplifier 25, 26, 27, the final transistor 27 being connected so as to load a high-impedance current source 50, 51, Fig. 2 connected to terminals 11, 12. The potential difference between the bases of transistors 19, 18, Fig. 1, required to maintain a given ratio between their emitter-current densities, is proportional to the absolute emitter-junction temperature and a logarithmic function of the emitter-current density ratio. If the collector currents of transistors 19, 18 are maintained equal the emitter currents will also be substantially equal, and transistor 18, having the smaller junction, will carry four times the current density of transistor 19. Any deviation from this ratio will be sensed by the amplifier 25-27, the loading on terminals 11-12 being thereby adjusted to vary the PD between 11-12 and thus the PD between the bases, substantially to restore the ratio to a constant amount. At 300 K the voltage required between terminals 13 and 14 is 36 mV, the chain resistances 15-17 being so arranged that the corresponding voltage between terminals 11 and 12 is 3 volts. A non-linear relation between the voltage across terminals 11-12 and the temperature may be provided by connecting suitable components between one of terminals 11 and 12 and one of terminals 13 and 14 thereby to shunt one or more components of the resistance chain 15-17. The circuit of Fig. 4, for example, provides a piecewise-linear approximation to a reducing-gradient curve, illustrated in Fig. 5. In a second embodiment, Fig. 12 (not shown), the transistors (18<SP>1</SP>, 19<SP>1</SP>) are chosen to have the same junction area, their collector currents being maintained in the ratio 4:1 by the choice of inverter transistors (22<SP>1</SP>, 23<SP>1</SP>) having junction areas in corresponding ratio. The circuit may conveniently be made as a monolithic I.C.